The present invention relates to apparatus for measuring a surface characteristic of a component such as a workpiece. The invention has particular but not exclusive application in the measurement of the form or profile of a surface, where form or profile is understood to relate to deviations from linearity with irregularities having wavelengths of the same order as the dimensions of the surface; for example the surface may be spherical or cylindrical or may bow inwardly or outwardly.
Taylor Hobson Limited of Leicester, England manufacture a range of metrology instruments and in particular the publicly available xe2x80x9cForm Talysurfxe2x80x9d range of instruments for measuring the surface form or profile of an object or component.
In the Form Talysurf instrument, a stylus carried by a support arm is mounted such that it can pivot in order to allow a stylus tip to follow the surface of a component during relative movement of the stylus and the surface. The displacement of the stylus as it follows the surface is detected using a transducer and the output of the transducer is logged by data logging and processing equipment (e.g. a personal computer) together with data indicating the position of the tip of the stylus along the component in order to provide data giving a measurement of the surface profile or form of the component. The stylus or the surface or both may be moved so as to allow the construction of a two or three dimensional surface map which can then be displayed on a visual display unit or printed as a hard copy using a variety of known techniques.
One type of position transducer typically used in connection with such a stylus in order to detect the position of the stylus tip as it follows the surface is a linear variable differential transducer (known as a LVDT).
The range of movement within which measurements may be taken by such apparatus is limited This makes such apparatus unsuitable for use in the measurement of the surface form or profile of components or objects having a relatively large (tens or hundreds of millimeters) variation in surface form.
In one aspect, the present invention provides a metrological instrument wherein a surface sensing probe is maintained within an operational range using first, for example, optical sensing means and axial displacement of the sensing probe in response to surface characteristics of the surface being sensed during relative movement between the surface and the probe is determined using a separate measurement system which may, for example, comprise an optical interferometer. This enables the surface sensing probe to rake measurements over a relatively large range.
In one aspect, the present invention provides a metrological apparatus for measuring surface characteristics of a surface of an object such as a workpiece, comprising: an elongate stylus support member for carrying at one end a stylus for sensing a surface of an object; means for effecting relative movement between the support member and a surface to be measured to enable a stylus carried by the support member to traverse the surface; a carriage carrying the stylus support member and enabling a stylus carried by the support member to be displaced relative to the carriage in a direction parallel to its length in response to variations in the surface being sensed as a stylus carried by the stylus support member traverses the surface; first sensing means for optically sensing the displacement of the stylus support member relative to the carriage and for providing an output signal representative of the position of the stylus support member relative to the carriage; control means for moving the carriage in response to the output signal of the optical sensing means so that a predetermined positional relationship exists between the carriage and the stylus support member; and second sensing means for sensing the displacement of a stylus carried by the support member as the stylus traverses a surface being sensed This enables measurement of the form or profile of a surface whose surface form varies over a range greater than the operational range of the stylus to be achieved because the first sensing means and control means act, in use, to maintain the stylus within its operational range regardless of the actual distance the stylus has moved in the measurement direction as it traverses the surface being measured. Such features enable the apparatus to be of compact and simple construction. Additionally the apparatus should be less sensitive to external variations such as temperature.
In one aspect the present invention provides a metrological apparatus providing an air bearing for supporting a surface sensing probe to allow displacement of the probe as it follows a surface being sensed during relative movement between the surface and the probe, the apparatus having means for maintaining the probe within an operational range in relation to the air bearing and separate measurement means for determining the displacement of the probe as it follows the surface being measured. In such an aspect, the air bearing provides low stiction (static friction) which improves repeatability. At least one of the maintaining and measurement means may be an optical sensing system, such as an optical interferometer. In such a case a greater range and resolution and a wore linear response than an LVDT can be provided.
In one aspect, the present invention provides a metrological apparatus comprising a turntable for supporting a workpiece, and a gantry carrying a carriage for supporting a sensing probe above the turntable to allow the probe to move towards and away from the turntable, means being provided for allowing relative measurement between the turntable and the probe so as to cause, in use, the probe to traverse a surface of a workpiece carried by the turntable. For example, the turntable may be mounted so as to be slidable beneath the gantry. This enables the sensing probe to traverse different measurement paths across the surface with the different measurement paths being at an angle to one another and with the angle being determined by the degree of rotation of the turntable between measurements. This enables, for example, a three dimensional profile of a non-rotationally symmetric component or object such as an aspheric lens to be produced. The carriage may comprise an air bearing for the probe. The probe may comprise a stylus support for carrying a removable stylus. Means may be provided for maintaining the probe in an operational range with respect to the carriage independently of a measurement system for determining the displacement of the probe as it follows the surface.
In one embodiment, the present invention provides a metrological apparatus for measuring surface characteristics of a component the apparatus comprising:
a probe carriage for supporting a probe to allow the probe to move axially relative to the carriage so as to follow a surface being sensed;
moving means for causing relative movement between the probe carriage and a component to allow a probe carried by the carriage to traverse a surface of the component;
optical sensing means for providing a signal indicative of displacement of the probe relative to the probe carriage;
control means for controlling movement of the probe carriage in said axial direction in response to the signal provided by the optical sensing means to maintain the probe in an operational range relative to the probe carriage; and
further sensing means for providing a measurement of displacement of the probe as it follows a surface during use of the apparatus.